EVALUATION AND TESTING OF NAIL LACQUERS 333 the problem of ease of application is the rate of drying of the lacquer. If it dries too rapidly, even appli- cation is difficult. If the film dries too slowly it may be necessary to wait too long before applying the second coat and it may be too long before the hands may be used after applying the lacquer. The rate of drying of a new lacquer may be compared with that of a standard lacquer by making simultaneous pours on glass and observing the time required for each to become dry to the touch. In order to make a truly valid comparison the viscos- ity and the total solids of the lac- quers should be the same in order that films of equal thickness may be formed. For absolute comparison this test should be carried out under conditions of constant tem- perature and constant humidity. Constant temperature and humidity rooms are not usually found in cosmetic laboratories. However, relative drying rates may be ob- served in the ordinary laboratory by making simultaneous pours. The drying time observed on the glass plate pours will be very different from that actually found when the lacquer is applied to the nails. The glass plate drying time will be six to eight times as long as the time required for the same polish to dry on the nails. The more rapid rate of drying on the nails is due to effect of the body heat. The nails are of course at body temperature at all times. For this reason a check of the drying time of the new lacquer compared with the standard lac- quer when applied to the nails should also be made. The gloss of a polish is another factor which the customer takes into consideration when com- paring two nail polishes. In the laboratory the pours which have been made on glass plates for the purpose of comparing drying times may also be used to determine the relative gloss of two nail lacquers. The Hunter gloss inspection lamp can be used to give usef•ul com- parative values for the gloss. The most important charac- teristic of a nail polish as far as the customer is concerned is its wear resistance. Unfortunately wear re- sistance is not a function of any one nail polish characteristic. Several factors are involved in the general attribute called wear resistance. In use the polish must withstand excur- sions into the depth of my lady's hand bag, trips into the office fines, and general contact with objects in the environment. There is no question but that the hardness of the film is a factor in the wear resistance. However, it does not follow that a harder film will give better wear for the film must also have the proper flexibility. Harder films are usu- ally less flexible and may there- fore give poorer wear. A Sward Rocker may be used to test the rela- tive hardness using the same films which were used for observing drying time and gloss. Not too long ago a survey was published showing the relative value

334 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of various nail polishes. The rat- ings were largely based on abrasion resistance as determined by allow- ing a fine stream of sand to impinge on a polish film poured on glass. The weight of sand required to wear through the film is taken as a measure of the abrasion resistance. Unfortunately there is little correla- tion between the abrasion resistance measured in this manner and the wear resistance of the polish on the nail. This may be due to the fact that few wearers of nail polish build castles in the sand. The polish does not usually fail by abrasion but it often fails by chipping which is probably caused by the bending of the nails and the failure of the adhesion bond be- tween the polish and the nail. This adhesion between the nail and polish film is an important factor in the wear resistance of any polish. Un- fortunately this adhesion is very difficult to measure. However, fair correlation has been found between the adhesion of a lacquer film to a glass plate and wear resistance. The same pours on glass used for observing gloss and hardness may be used for the ad.hesion test by testing them with a knife or scapel. Small differences in adhesion are quite easily noted by an operator who has tested a few such pours. Unfortunately there has been found to be little correlation be- tween the laboratory tests and actual wear resistance of the polish as used by the customer. This is an indication of the complexity of the problem with which the producer of nail lacquer is faced. His task is quite different from that which confronts the makers of lacquers for other applications. His principal problem resul{s from the fact that the nails are living tissues. This means that the surface to which the lacquer is applied is different, not only from person to person, but also it may change on the same person due to variations in the physiological condition of the individual. This sort of prob- lem does not confront the maker of a lacquer to be used on brass, or steel, or other materials which have constant surface properties. This means that the final evalu- ation of a nail lacquer can only be made by actual wear tests on the nails. The wear testing of nail polish also presents problems. However, it has been found that a comparative evaluation can be made by applying the polish under test to the alternate right and left hands of about a dozen girls and a stand- ard polish of the same shade to the other hand of each. After four days a comparison is made between the two hands of each subject and in this way the relative wear resistance of each polish may be determined. The polish is applied to both left and right hands because the right hand usu- ally is used more than the left. This test will not show up small differences in wear resistance, but will bring to light wide differences. Small differences can only be de- tected by conducting wear tests on a large number of subjects and mak-